In a mechanical seal used as a shaft seal for a centrifugal pump, etc., burn-in or breakage of its sliding surfaces may be caused at the time of start-up, since there is no fluid between the sliding surfaces of the mechanical seal due to temporary negative pressure in a stuffing box where the mechanical seal is installed. To solve such a problem, for this type of the mechanical seal, a double mechanical seal (which may be simply referred to as a double seal) is conventionally mounted, and quenching liquid is flowed between two seals, by which the sliding surface is prevented from being dried. Note that the double seal is often used, for example, in the case of preventing inside fluid from leaking outside, or in the case that the inside fluid can be crystallized if it is dried.
As a way of applying the double seal into the shaft seal of the centrifugal pump, etc., a mechanical seal wherein a gland packing is conventionally used may be modified. In this case, the mechanical seal is mounted by using a space in the stuffing box (refer e.g. Japanese Utility Model Publication No. H07-3094 (Patent Document 1)).